MICROPLANKTON 249 



spring. It can be seen that its distribution never overlapped that of F. karsteni during these two 

 surveys, and this emphasizes the extent to which the latter can be regarded as an ' indicator ' of inshore 

 conditions. 



Two abundant 'Pennatae' have not been dealt with in this series: Nitzschia delicatissima and 

 N. seriata. They tended to be irregularly distributed, perhaps reaching their greatest relative 



20" 



25°H 



S 



o 



u 



T 

 H 



o 



30- 



SURVEY- 1 1 



NIL 



<-o 5 n 



T 



MO WE POINT « 6 



iotio 

 >io 6 



WALVIS BAY 



SYLVIA HILL 



ORANGE 

 R. 



10° 15" EAST 



Fig. 89. Distribution of Asterionella japonica, survey II, 

 September-October 1950. (Station numbers are shown 

 in Fig. 2.) 



20 e 



25< 



O 



u 



T J 

 H 



30= 



fRAGILAKIA > | q* 



KARSTENI 



SURVEY:! 



ORANGE 

 R. 



IO° 15° EAST 



Fig. 90. Distribution of Fragilaria karsteni and F. granula'a, 

 survey I, March 1950. (Station numbers are shown in 

 Fig. 1.) 



importance in outer shelf- waters. Since they are among the most completely cosmopolitan of all 

 marine plankton diatoms and, moreover, very difficult to separate with certainty when working at the 

 speed necessary to obtain counts, it is scarcely surprising that they did not show such well-defined 

 distributional trends as most other members of the group. 



Figs. 92 and 93 show the relative importance (percentage of total diatoms) of Thalassiothrix longissima, 

 the most essentially oceanic of the ' Pennatae ' in this area, to be considerable at the offshore stations, 

 where the total phytoplankton was poor. It was equally abundant at some rich shelf stations where, 

 however, its proportion of the whole flora was insignificant. 



This same method of treatment was also found necessary to show the high relative importance of 

 the solenoid group offshore during the second survey only. Here they were definitely dominant, and the 



